Power is traditionally supplied to avionic equipment via a power cable at a non-null electrical potential and by return through the structure of the airplane by means of a metallic fixture to which the equipment is attached, at the null or grounding potential of the airplane.
The use of composite conductive structures is becoming more and more important in aircraft, with some such structures replacing metallic structures.
In the case of composite conductive structures, e.g. structures containing carbon and/or laminated aluminum/composite, the materials that, although they are conductive, do not exhibit the conductivity properties required in order to guarantee a sufficient return, and which may be sufficiently resistant to cause a consistent drop in voltage along the length of the current, overheating of the structure, or a fire hazard.
These structures, which do not constitute primary electrical structures, should not be used for current returns; the current return must be insulated from the conductive composite structure.
An additional problem is the fact that, if one chooses to manufacture a cable-based current return, it is necessary to configure the dimensions of the cable based on the distance to be traversed and the number of avionic devices connected, if the return cable is jointly used by more than one avionics, to provide several cables if a separate return is provided for, which complicates the design of electrical systems, increases the weight and cost of the system, and can be detrimental to the reliability of the system due to the complication of the power grid.